Injection pumps driven by the crankcase pressure utilize the crankcase pressure which changes during operation of the two-stroke engine. The pressure in the crankcase is dependent upon the rotational speed and load of the two-stroke engine. An overpressure develops with a downward movement of the piston in the direction toward bottom dead center; whereas, the pressure in the crankcase drops to an underpressure with the following upward movement of the piston toward top dead center. The crankcase pressure then swings between positive and negative values with the positive values likewise increasing to a maximum with increasing speed which then remain constant up to the highest speed. The pressure oscillations lie, for example, between approximately 0.75 bar and -0.2 bar.
U.S. Pat. No. 4,700,668 discloses a fuel injection pump wherein the pulse chamber is directly charged by the pressure present in the crankcase, while a mean pressure is supplied to the return chamber. The pump piston is moved upwardly and downwardly in correspondence to the pressure changes in the crankcase and thereby injects fuel into the combustion chamber of the two-stroke engine.
The air charge for the combustion chamber is reduced at high rotational speeds of the two-stroke engine because of the increasing resistance to flow in the air channels. For this reason, the quantity of fuel should be reduced at high speeds which is intended to be achieved with the aid of the crankcase mean pressure acting in the return chamber. However, this solution is not itself adequate to adapt the pumped quantity of fuel with sufficient accuracy to the rotational speed.
The spring which returns the membrane is configured as a helical spring which leads to a fuel injection pump which must be constructed so as to be large in the longitudinal direction of the pump piston.